Sanitary napkins are conventionally made of layers of cellulosic material such as wood pulp fluff, creped wadding or the like. These materials provide relatively good absorbency and some shape recovery when dry. Unfortunately, when these napkins are wet, the cellulosic material collapses, loses its shape and is much less comfortable to the wearer; with the result that even the low levels of shape recovery and flexibility present in the cellulosic material in the dry state tend to disappear.
Recognition of the discomfort problems associated with these napkins, both in the wet and dry form, has produced a variety of solutions. One of these solutions involves the addition of so-called super-absorbent materials to a sanitary napkin with the concomitant decrease in conventional cellulosic absorbents. This has resulted in a thin napkin which distorts easily and which suffers from loss of shape recovery.
Another solution which has been discussed in the patent literature involves the concept of adding thermoplastic material to the absorbent matrix. The thermoplastic is typically added as fibers or powder. It has been intermingled with the absorbent to keep the capillaries within the absorbent open, and at varying levels with decreased amounts present in the center or most absorbent area of the absorbent matrix. In the latter case, thermoplastic is added to establish a capillary gradient in which absorbent layers with larger capillaries are present nearest the body of the wearer.
Examples of the addition of thermoplastic can be found in: U.S. Pat. Nos. 4,082,886 and 4,129,132 issued to George A. M. Butterworth et al.; 3,976,074 issued to Harry G. Fitzgerald et al.; 4,054,141 issued to Julius Schwaiger et al.; 4,047,531 issued to Hamzeh Karami; 3,545,441 issued to Gunnar Gravdahl and 4,219,024 issued to Donald Patience et al.
The prior art listed above recognizes the desirability of having a capillary gradient with larger capillaries nearest the body of the wearer and smaller capillaries in the area where maximum absorbency is desired. If the absorbent matrix in the napkin was designed so that the smallest capillaries were closest to the body of the wearer, fluid would remain in the area of the smallest capillaries and would not disperse through the rest of the absorbent matrix due to capillary attraction. If a number of large capillaries are near the point of fluid contact and a greater number of small capillaries are farther from that point, then fluid will be drawn away from the initial contact site.
In general, the greater the level of unfused thermoplastic material, the higher the amount of shape recovery introduced into the absorbent matrix of the sanitary napkin.
A napkin with a relatively thick layer of large capillaries readily transfers relatively low viscosity fluids such as urine, water, or even blood. It has been recognized, however, that menses is a complex fluid with uterine blood being only one component of its composition. Additionally, menses contains cellular debris and a mucus-like fraction. It has also been recognized that the character of menses can differ in composition, viscosity, volume and flow rate from individual to individual and also at different times in the menstrual cycle with a given individual. It has also been determined that certain women have consistently high viscosity menses and comparatively low flow volumes.
Since highly viscous menses has been identified, it has now been discovered that the composition of menses also has a significant effect on the transport of fluid from the cover into the absorbent matrix of a sanitary napkin. When the viscosity and cellular debris content of menses are relatively low, usually during periods of high flow, it passes relatively rapidly through the cover material and into the absorbent matrix; even in cases where the portion of the absorbent matrix adjacent the cover material has relatively large capillaries. However, it has been observed that more viscous, high debris-content menses, especially prevalent during periods of low flow, tends to stay on the upper surface of the cover. In addition, particularly when there is little capillary attractive force exerted on the bottom of the cover by the absorbent matrix, the fluid component tends to stay on the surface and then run off the sides of the napkin. It is believed that earlier attempts at designing sanitary napkins containing thermoplastic materials have been unsuccessful because of the failure of the absorbent matrix to draw viscous menses through the cover material.
U.S. Pat. Nos. 4,214,582 issued to Harish A. Patel and 3,285,245 issued to Charles L. Eldredge et al describe wound dressings in which a thermoplastic fluid pervious cover is fused to an absorbent layer also containing thermoplastic. The Patel patent discloses a wound dressing having a hydrophobic cover, a microcreped absorbent layer underneath the cover and a second hydrophobic cover positioned on the bottom of the absorptive layer. This combination is subjected to embossing by a compactor to provide large locally fused discrete portions along the cover surface in contact with the wound. This relatively severe compression in fusing is designed to provide an irregular top surface for ease of release from the wound area as it heals. The fusing of the major proportion of the top surface area also provides a suitable release surface.
The Eldredge patent discloses a surface in which the fusible fibers are drawn through the cover material to provide a soft matt finish and aid in the wicking of fluid wound exudate along the top surface of the cover and into the main absorbent area through the cover.
Neither of these configurations would be particularly useful for a sanitary napkin. In the wound dressing described by Patel, the rapid discharge of fluid would settle on the relatively large fused surface since there is no penetration of fluid in the fused area. The relatively great amount of compression applied over relatively substantial areas in Patel also tends to destroy such deformability and compressibility as may otherwise have been present. To design a napkin having the Eldredge configuration would tend to provide a competing capillary structure above the cover which would tend to trap debris and prevent fluid transfer into the napkin resulting in a perpetually wet surface after intitial discharage.